Editor’s Note: I’ve written about Rob Dumont in the past. He is one of the fathers of the green building movement, and I’m very pleased to have permission to reprint one of his columns here.
By Rob Dumont
Have there been any recent advances in the area of passive solar heating for residences?
Passive solar heating has been known since the time of Socrates. However, it is only slowly catching on in Canada. Improved windows have made the recent advances possible.
In the 1970s there developed two schools of thought regarding how best to reduce space heating bills in residences using passive means. One school was called the “mass and glass” school. With this approach, large south facing windows would be used along with concrete slabs and heavy construction such as adobe. This school had a lot of proponents coming out of the south-western United States. New Mexico was a hotbed. New Mexico has an especially favourable solar energy climate and relatively mild outdoor temperatures compared with most of Canada. A photo of the David Wright Home in Santa Fe, New Mexico, is shown in Figure 1. In Figure 2 a cross section of the house is shown.
Figure 1. South exposure of the David Wright House in Santa Fe, New Mexico
(Photo Credit: Design for a Limited Planet, Skurka and Naar, 1976)
Figure 2. Cross Section of the David Wright Mass and Glass House
( Credit: Design for a Limited Planet, Skurka and Naar, 1976)
Note the large amounts of mass in the concrete floor and gravel beneath, and in the adobe walls for the house. Note also the insulation on the outside of the thermal mass. The mass and glass approach generally worked well in that climate. However, when translated to other climates, the approach did not work as well. There was a monitoring study done in Minnesota of a number of passive solar houses in the 1980s and it was found that the more south glass a house had, the higher the heating bill! There were two main reasons that the energy bills for the Minnesota houses with large double glazed south windows were higher. First, Minnesota is further north and considerably more cloudy then New Mexico. And second, Minnesota is considerably colder than New Mexico.
The other passive school was known as the “light but tight” school. This approach generally did not use additional thermal mass inside the house. However, these houses did use modest south facing window areas, high insulation levels, well sealed construction and heat recovery ventilators. The Saskatchewan Conservation House was an early example of this approach. A photo of the Conservation House is shown in Figure 3. In Figure 4, a cross section through the Conservation House is shown. Note that there was no additional thermal mass in the house.
Figure 3. Saskatchewan Conservation House, 1977-78
An early “light but tight” passive Canadian House. Note the modest south window area
compared with the David Wright House in Santa Fe.
Figure 4. Cross Section of the Saskatchewan Conservation House. Note that no additional thermal mass was used. The heat storage tank was used for the active solar system.
Since the 1980s, window technology has advanced considerably. We now have triple glazed windows with low emissivity coatings, argon gas fills, low conductivity spacer bars, and relatively high solar transmittance. For instance, it is now possible to buy windows of this type with a solar heat gain factors as high as 0.59 with R values in the R 5 range. Given that windows have improved so much in performance, it is now time to revisit a combination of “mass and glass” and “light but tight”.
Typically in light wood frame construction one should have no more than about 6% ratio of south window area to floor area. If the south window area is greater, the house will overheat from the high solar gains, particularly in the fall. Usually the worst time in Canada for a passive solar house to overheat is in late September/early October. At that time of the year the sun is lower on the horizon and more solar energy passes into the south windows; in addition it is usually relatively warm at that time of the year in Southern Canada. This combination of a large solar energy input and warm outdoor temperature usually results in overheating at that time of year unless the house is carefully designed.
If, however, additional thermal mass is present in the home, the rise in temperature in the home is manageable. As a simple example consider a home with an internal heat storage capacity of 25 megajoules per degree Celsius. This value is the value that we measured on the Saskatchewan Conservation House in Regina way back in 1978. In a typical house the interior gypsum board and the wood framing members make up the main heat storage elements. Assume a day when the outdoor temperature is the same as indoors, as might occur in October. If 250 megajoules of energy pass through the south facing windows, the temperature in the house will rise 250/25 = 10° C. A 10° C temperature rise is outside the comfort zone for virtually everyone. This is the amount of solar energy passing through the windows in October on a sunny day at latitude 48 degrees, assuming 20 square metres of south glass with a solar heat gain coefficient of 0.59. If, however, the internal heat storage capacity of the house were raised to 50 megajoules per degree Celsius, the rise in temperature of the house will be only 250/50 or 5° C. (This assumes that the mass is fairly thin and not, for example, in a very thick concrete slab.)
Additional thermal mass is desirable in a home as a means of storing energy between day and night and also over longer periods of time.
There are several relatively inexpensive ways of adding thermal mass to a house. In my own home this was done by placing the scrap gypsum board into the hollow interior stud cavities in the house. An architect friend carried this further by gathering scrap gypsum board from other houses and stuffing it into the hollow cavities between the floor joists in his home. This technique can be used in most homes, but one should be careful about placing too much weight on the floor joists in a home. Consult a structural engineer. Another technique that has been used is to place a concrete topping on the wood joist floor system. A thickness of 2.5 inches of concrete will add an additional weight of about 30 pounds per square foot of floor area. On a house with 2000 square foot of floor area, this would add 60,000 pounds of additional mass to the house. The thermal capacity of that concrete would add about 18 Megajoules per degree C to the heat storage capacity of the house.
If the house is a slab on grade design, the concrete floor can act as a thermal mass. It is very important, however, to add rigid insulation boards beneath the entire concrete slab. Less expensive beadboard (expanded polystyrene, usually white in colour) can be used instead of extruded polystyrene (usually blue or pink). Depending on the location in Canada, R values in the range of R15 and higher should be used.
The technique of adding additional thermal mass by way of concrete toppings was recently used in a house in Edmonton. If you go to the web site for the Edmonton Mill Creek House (http://greenedmonton.ca/MillCreekNetZeroHome) there is a good description and photos of the technique that was used. The house also used high quality Duxton windows with a good solar heat gain factor and high R values.
Figure 5. The Mill Creek Net Zero House in Edmonton, AB
Mass, good glass, superinsulation, air tightness and an HRV
(Photo Credit: Mill Creek House Web Site)
A picture of the Mill Creek house is shown in figure 5. As can be seen, the south side has a lot of glazing. To save money, the 2.5 inch thick concrete topping also serves as the finish floor for the house. A staining technique was used to enhance the appearance of the concrete floor. If the house did not have this additional thermal mass, it would be very prone to overheating because of the large south window area. The house is very well insulated with R 100 attic insulation, R 56 walls, R25 basement floor, air tight construction and a heat recovery ventilator. The house also has a photovoltaic system and a solar domestic hot water heater. The house will be occupied shortly.
The “mass and good glass” approach will work best in the sunnier parts of Canada. However, in Canada we do have some notoriously cloudy locations such as Prince Rupert where the approach might not work well.
A computer model such as HOT-2000 can help greatly with analyzing where the passive solar approaches will work best. Great south windows with a high solar heat gain factor and high R values are crucial in the Canadian climate to make a great passive solar design. The “boring” parts like superinsulation, air tightness, a good HRV are also crucial.
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